<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>dom2</title>
</head>

<body>
  <button id="btn1">点击一下</button>
  <script>
    var btn = document.getElementById("btn1")
    function gongneng1() {
      console.log("旧功能");
      console.log(this);
    }
    function gongneng2() {
      console.log("新功能");
      console.log(this);
    }
    btn.addEventListener('click', gongneng1)
    btn.addEventListener('click', gongneng2)
    // 解绑操作
    btn.removeEventListener('click', gongneng1)
  </script>
</body>

</html>